The human body functions through a number of interdependent physiological systems controlled through various mechanical, electrical, and chemical processes. Electrical signals cause a heart to beat. In a healthy patient, regular heart beats pump blood through the cardiovascular system. The human cardiovascular system is responsible for receiving oxygen-deprived blood into the heart from the venous system of the body, delivering the oxygen-deprived blood to the lungs to be replenished with oxygen, receiving the oxygenated blood from the lungs back into the heart, and delivering the oxygenated blood to the body via the arterial vasculature. The respiratory system, through the breathing mechanism, performs the function of exchanging oxygen and carbon dioxide with the external environment.
Various disorders may affect the cardiovascular, respiratory, and other physiological systems. Breathing disorders include various forms of rhythm-related disorders such as sleep apnea and hypopnea. Disordered breathing is a respiratory system condition that affects a significant number of the population, particularly between the ages of 30 and 60 years.
Various types of disordered respiration have been identified, including, apnea (interrupted breathing), hypopnea (shallow breathing), tachypnea (rapid breathing), hyperpnea (heavy breathing), and dyspnea (labored breathing). Combinations of the respiratory cycles described above may be observed, including, for example, periodic breathing and Cheyne-Stokes respiration (CSR). Cheyne-Stokes respiration is particularly prevalent among heart failure patients, and may contribute to the progression of heart failure.
Sleep disordered breathing is particularly prevalent and is associated with excessive daytime sleepiness, systemic hypertension, increased risk of stroke, angina and myocardial infarction. Disordered breathing can be particularly serious for patients concurrently suffering from cardiovascular deficiencies.
Various aspects of cardiac activity (e.g., heart rate, arrhythmias) can be detected by measuring, recording, and analyzing cardiac electrical signals, such as an electrocardiogram (ECG) signal. One way of measuring ECG signals involves attaching electrodes, typically ten, externally to a patient's skin and sensing the electrical signals that form the ECG waveform.
Implantable monitoring systems can be implanted under the skin with electrodes that sense subcutaneous electrical signals, including ECG signals, which are analyzed as being indicative of cardiac activity. In such systems, the electrodes also receive extraneous non-cardiac electrical signal information, which is typically filtered out to produce a more readable ECG. Non-cardiac electrical signals can be generated by muscle tissues during physical activity, for example. In some examples, an implantable loop recorder (ILR) can record and quantify patient heart electrical activity.